The torque and power handling capability of any gear transmission is limited primarily by the number of teeth, the strength of the teeth and the Hertz pressures which become active during the conveyance of motion and power from the input to the output side of the transmission. In a typical spur gear transmission, only one pair of teeth is engaged at any time. On the other hand, planetary gears of the general type of interest here typically divide the flow of power over three or more sets of gear teeth. In the latter type transmission, the more teeth that are engaged, the more critical the manufacturing tolerances have to be in order to assure simultaneous and equally loaded contacts for all gear teeth in the different sets. Thus the structural rigidity and dimensional aberrations prevailing with the interlocking gear elements in a transmission determine ultimately the number of teeth which are actively engaged at any given time. This limitation is well known for planetary gears as well as for worm gears. To applicant's knowledge, however, that problem has not been alleviated in the case of the toroidal transmissions of the type with which we are concerned here.
Toroidal transmissions have been known for some time. In one such transmission disclosed in my U.S. Pat. No. 3,174,354 the grooves are in the form of slits which guide strip-like gear elements. Those gear elements require spokes to support the strips which must twist to accommodate the varying lead angles of the grooves in which the gear elements slide, whereas for meaningful torque transmission, those spokes should be stiff and strong in that direction to carry the load. Resultantly, that type transmission is only useful as a motion transducer, for example, to turn the hands of a clock. It does not have any real ability to transmit torque or drive a heavy load.
In other transmission of this general type shown in my U.S. Pat. No. Re. 26,476, the grooves in the housing have a circular or gothic arch profile for the purpose of guiding gear elements in the form of circular ball bearing units. Those bearing units are rigid so that there is no means to distribute the load among the gear elements without imposing excessively tight tolerance requirements on the diameters and other dimensions of the drive worm and stator, as well as on the diameter of the mounting ring required to support the bearing units. As a practical matter, because of inevitable dimensional variations, even if all the teeth on the bearing units are carefully honed to minimize pressure points between the teeth and the races, it is next to impossible to assure simultaneously an equally loaded contact for all the bearing units in the transmission. Rather, the load is concentrated essentially on one bearing unit so that advantages of high efficiency, long transmission running life and small overall size and weight are not attained. Rather, the transmission must be overdesigned for its particular end use.
In fact, it is a characteristic of all prior gear systems of which I am aware, particularly planetary systems, that the gear elements have rigid teeth ostensibly to maximize their load carrying capabilities. Therefore, they all share those very same problems.
My prior transmission is disadvantaged also because the teeth of each bearing unit ride in different stator races or grooves. For example, if each unit has twelve teeth, twelve different grooves are required. In that event, a given tooth runs in groove convolutions 1, 12, 24, 36, 48, 60, 72, 84, 96, 108, 120, 132, 144. Thus the ratio is divided into 144/12. The angular spacing between the bearing units to achieve this result is fixed as described in that patent. Likewise, if the units each have six teeth, six grooves are required yielding a ratio divided into 72/6. In other words, the reduction ratios available with that prior drive are limited because of the assumption that the number of stator grooves and the number of teeth must divide evenly.